The invention is related to a method of producing ultra-fine grain structure for unalloyed or low-alloyed steels. The steels are usually of the hypoeutectoid type, but may be also of the eutectoid type.
Unalloyed and low-alloyed steels are the most significant group of the metals used by the industrialized world. Their properties vary according to carbon content, alloying element contents and the treatments included in the steel manufacturing. Strength, toughness and weldability are the most important properties of low-carbon steels (C less than 0.25%), and therefore they are widely used in various structures. The widespread use of medium-carbon steels (C=0.25-0.60%), e.g. quench and tempering steels, is based on their high strength and good toughness. Their weldability is poor, however, due to the tendency to hardening caused by rather high carbon content. High-carbon steels ( greater than 0.60%) are harder and more resistant to abrasion, but their toughness and also weldability are poorer than in steels with lower carbon content.
The iron-carbon phase diagram for carbon contents of 0 to 1.0% is presented in FIG. 1. During slow heating below the temperature Ac1, the structure of a steel is naturally ferritic (a-Fe) and/or pearlitic (a-Fe+Fe3C). Between the temperatures Ac1 and Ac3 the more austenite (g-Fe) in addition to ferrite is formed in the structure the higher the temperature is rising, and above the Ac3 temperature the structure is fully austenitic. With slow heating, the Ac1 temperature is about 730xc2x0 C., and the Ac3 temperature is varying according to carbon content. The Ac3 temperature of pure iron is about 910xc2x0 C., of steel containing 0.1% carbon about 880xc2x0 C., and of steel containing 0.75% carbon about 730xc2x0 C.
During conventional or fast cooling, the transformation of austenite to ferrite and pearlite is not beginning until at the temperature Ar3, which is tens or up to two hundred degrees lower than the Ac3 temperature. Correspondingly, the stopping temperature of the phase transformation, Ar1, is clearly lower than the Ac1 temperature.
In a steel containing more than 0.1% carbon, especially if it contains enough alloying elements increasing hardenability, e.g. manganese, chromium, nickel, or molybdenum, the transformation of austenite into ferrite and pearlite becomes slower and can also be hindered partially or completely by fast cooling. In the structure cooling down, also bainite and/or martensite are then formed at lower temperatures, these phases being stronger than ferritic-pearlitic structure but usually not as tough. In a very fast cooling, i.e. hardening, a fully martensitic structure is aimed at with medium-carbon or high-carbon steels.
Unalloyed and low-alloyed steels are often produced so that molten steel is casted, and then the slabs of an appropriate size are usually heated to 1200 to 1300xc2x0 C. and rolled thinner, the steel at the same time cooling down. Lastly, a plate, bar, etc. is allowed to cool down or is cooled with accelerated cooling to the room temperature. After hot rolling, some steels are further normalized or austenized for hardening above the Ac3 temperature. For example, a steel to be normalized is usually cooled down to 500xc2x0 C., only, from where it is heated in a furnace to a temperature of about 30 to 50xc2x0 C. above the Ac3 temperature (often within the range of 800 to 920xc2x0 C.) and then usually let to cool down.
Austenizing of medium-carbon and high-carbon steels before hardening is also accomplished above the Ac3 temperature, but with accelerated water or oil cooling the structure is hardened, i.e. changed mainly to martensite. A steel may sometimes be used in this condition for purposes in which good resistance to abrasion is required, although the toughness of the structure remains poor. If also good toughness is desired for a martensitic steel, it has to be tempered at a temperature of about 550 to 650xc2x0 C. Then a quenched and tempered (QT) steel is concerned which is very suitable for transmission axles, for example, for which both strength and toughness are required.
The strength and toughness properties of a steel can be improved by reducing the grain size of the microstructure. The grain size of the final ferritic-pearlitic structure is the smaller the smaller the grain size of the austenite is and/or the more deformed state the austenite has before cooling and phase transformation. Also the properties of bainitic, martensitic and QT structures will be improved in the same way as the grain size is reduced.
A small grain size is tried to get, for example, by adding small amounts, usually less than 0.1%, of microalloying elements, like niobium, titanium or vanadium, into a molten steel. Very small carbide, nitride and carbonitride precipitates of these alloying elements are then formed in the structure during the phases of steel production. Movement of grain boundaries is hindered by these small precipitates, and thus the grain growth at high temperatures is retarded. Steels alloyed with the above mentioned microalloying elements are often called fine-grained steels.
The grain size of a steel can be reduced also by an improved hot rolling, so-called thermomechanical rolling (TMCP). These so called TM steels are used for very demanding applications, e.g. bridge constructions, because, as low-carbon steels are concerned, the best combination of strength, toughness and weldability is achieved by these steels. TM steels are often also microalloyed steels.
Thermomechanical rolling is carried out at lower temperatures than normal rolling, i.e. below 1200xc2x0 C., and the rolling is finished near the Ar3 temperature, either a little above it the structure being still austenite or a little below it the structure already containing some ferrite, too. The grain size of austenite is about 20 xcexcm or larger before the last passes, and after rolling the worked grains are usually prolonged because no recrystallization of the microstructure occur due to the low rolling temperature.
Accelerated cooling to about 500xc2x0 C. after rolling and lastly slower cooling to the room temperature are often associated with thermomechanical rolling. In low-carbon and high-carbon steels, prolonged grains transform during cooling into ferrite and pearlite. As the ferrite grain size of conventionally rolled steels is 10 to 30 xcexcm, the grain size of TM steels is usually between 5 to 10 xcexcm and at its best 4 xcexcm.
Still smaller grain sizes of microstructure have been achieved by using various methods, whereupon steels with ultra-fine grain size may be spoken about. Mostly UFF (ultra-fine ferrite) steels have been dealt with. For different microstructures, it is difficult to determine the upper limit of ultra-fine grain size, but for ferritic steels it is in every case less than 5 xcexcm and preferably from 1 to 3 xcexcm. Pearlite and also bainite and martensite are formed in different ways than ferrite, and their grain sizes are typically a little larger, which is true also for steels with ultra-fine grain size.
A method combined with hot rolling of carbon or carbon-manganese steels with low carbon content is presented in U.S. Pat. No. 4,466,842 (Yada et al.), in which method heavy working is carried out during the final stages of hot rolling near the Ar3 temperature. The ferrite grain size obtained is about 4 xcexcm after a reduction of 40%, about 3 xcexcm after a reduction of 60%, and about 2 xcexcm after a reduction of 75% or more.
In some cases, heat treatment of steel may result in a grain size as small as 3 xcexcm. A method has been presented in the applicant""s international patent application PCT/FI98/00334, by which method, depending on the steel type and possibilities to carry out the heat treatment, a grain size of about 5 xcexcm, and even a grain size of up to 3 xcexcm with some steels and process parameters, can be achieved. The method usually necessitates fast or very fast temperature changes e.g. during heating and cooling, and therefore the realization thereof in practical production processes is often problematic.
An object of the present invention is to present a method which is simple and easy to realize and may be applied as widely as possible for producing an ultra-fine grain size for a steel.
A method according to the invention is characterized in that what is defined in claim 1 of the appended claims. In the other claims, various embodiments of the invention are defined.
The method according to the invention can be used instead of conventional thermomechanical treatments and fine-grain treatments or together with them for improving properties, especially the strength and toughness, of unalloyed or low-alloyed hypoeutectoid or eutectoid steels (carbon content not more than 0.8%). The necessary treatment can be carried out easily and with simple operations during the last stage of a conventional manufacturing process. Any special working methods or very strong working are not needed. After the treatment, the microstructure of a steel can include ferrite, pearlite, bainite and/or martensite.